A CCD image sensor typically includes an array of photosensitive areas that collect charge carriers in response to light striking each photosensitive area. For an interline transfer sensor, the charge is transferred from the photosensitive areas to vertical shift registers. The vertical shift registers shift the charge in parallel one row at a time to a horizontal shift register. The horizontal shift register then serially shifts the charge to an output circuit.
With a full frame transfer image sensor, the photosensitive areas also operate as vertical shift registers. The charge is shifted in parallel one row at a time from the vertical shift registers to the horizontal shift register. FIG. 1 is a simplified top view of a portion of a first full frame pixel array according to the prior art. Pixel array 100 includes multiple photosensitive areas 102. Electrodes or gates 104, 106 are disposed over the photosensitive areas in an alternating pattern. To shift charge through the photosensitive areas, a vertical driving pulse V1 is applied to electrodes 104 and a vertical driving pulse V2 is applied to electrodes 106. Typically the driving pulses V1, V2 are applied at both ends of the electrodes 104, 106.
Typically electrodes 104 are formed by a first polysilicon layer and electrodes 106 by a second polysilicon layer. Polysilicon is known to have a relative high resistance. This high resistance causes the waveforms of the driving pulses V1, V2 to deteriorate as the driving pulses V1, V2 propagate away from the ends of the electrodes toward the center or middle (represented by line 108) of array 100. This problem becomes worse as the size of the array increases.
Several different techniques have been developed to address the issue of signal deterioration across electrodes. For example, U.S. Pat. No. 5,194,751 includes a metal wiring layer that is connected to the electrodes through contact regions. FIG. 2 is a simplified top view of this second prior art construction. Contact regions 200, 202, 204, 206 between respective electrodes 208, 210, 212, 214 and the metal wiring 216 are arranged in an orderly fashion throughout the array 218. Instead of applying the driving pulses to the ends of the electrodes 208, 210, 212, 214, the driving pulses are applied to the metal wiring layer 216. Thus, the driving pulses are applied to electrodes 208, 210, 212, 214 at the various contact regions distributed throughout the array 218. One concern with this design is that the orderly contact pattern is susceptible to detection by human eyes when a CCD image sensor is illuminated by a bright light. This is due to interaction between the contacts between layers and the light.
Another concern is the density of contact regions 200, 202, 204, 206. In recent years, the trend of image sensor design is to increase the number of pixels and to shrink the pixel size. This means contact regions 200, 202, 204, 206 are formed closer together, increasing the probability for adjacent contact regions to short together. As shown in FIG. 2, each pixel 220 in array 218 includes one contact region. Thus, in each row of pixels, a contact region in one pixel 220 is immediately adjacent to, and diagonally offset from, the contact region in a neighboring pixel. This pattern can increase the percentage of—electrical shorts between electrodes. Electrical shorts between the contact regions reduce the manufacturing yield of image sensors and increase the cost to produce image sensors. On the other hand, if the density of the contact regions is too sparse, the benefit of reduced signal deterioration decreases because the driving pulses must propagate a greater distance between contact regions on each electrode.
FIG. 3 is a simplified top view of a portion of a third full frame image sensor according to the prior art. Two electrodes 304, 306 are disposed over each pixel 302. Electrodes 304 are connected to metal strips 308 via contacts 310, and electrodes 306 are connected to metal strips 312 via contacts 314. The driving pulses are applied to pads 316, 318 and to electrodes 304, 306 at the various contacts 310, 314 distributed throughout the array 300. This reduces the propagation delays of the driving pulses to the middle or center of the electrodes 304, 306.
As shown in FIG. 3, four adjacent metal strips are connected to the same pad 316, 318. This is an improvement over the FIG. 2 construction in terms of reducing electrical shorts between adjacent contacts because any short between adjacent metal strips within the four strips is not an issue since the four strips are all connected to the same pad. Another feature of the FIG. 3 design is that within each four-by-four (4×4) block 320 of pixels 302, the contacts within that block 320 are connected to the same electrode. Pixel array 300 has only one contact to a respective electrode in each single row and each single column in block 320 and any two contacts are not adjacent to each other. By reducing the contact density, the FIG. 3 design reduces the frequency of the shorts. However, since each contact is placed diagonally across pixel array 300 (as shown by the three diagonal arrows), the pattern of the contacts in array 300 is still susceptible to detection by the human eye when illuminated by bright light.
FIG. 4 is a simplified top view of a portion of a fourth full frame pixel array in accordance with the prior art. Pixel array 400 includes electrodes 402, 404 disposed over each pixel 406 in array 400. Contacts 408, 410 connect respective electrodes 402, 404 to a metal strip (not shown in FIG. 4). All of the contacts 408 disposed in column group 412 are connected to electrode 402, while the contacts 410 in column group 414 are connected to electrode 404. Within each five-by-five block 416 of pixels in column group 412, a contact 408 to electrode 402 is formed in row 1 and column 1; row 2 and column 4; row 3 and column 2; row 4 and column 5; and row 5 and column 3. This pattern of contacts in block 416 is fixed and repeats, or is tiled, across column group 412. The same contact pattern is used for the contacts 410 to electrode 404 in column group 414 (see block 418). The two contact patterns produce a diagonal pattern of contacts that can be easily seen (shown by two diagonal arrows in pixel array 400). The contact patterns produce image artifacts in an image captured by pixel array 400 when the pixel array is illuminated under a bright light.